PWM generation for DC/DC converters with frequency switching

ABSTRACT

A method for generating a pulse width modulation (PWM) control signal includes generating a sawtooth ramp signal at a first frequency under standard operating conditions using a ramp generator, generating a PWM square wave having a rising edge at a falling edge of the sawtooth ramp signal and a falling edge when the sawtooth ramp signal exceeds an error threshold, adjusting the frequency of the sawtooth ramp in response to a changed operating parameter of the ramp generator, and adjusting a peak input voltage of the ramp generator simultaneous with adjusting the frequency of the sawtooth ramp, thereby preventing one of a voltage overshoot and a voltage undershoot.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. provisional patent application61/737,858, filed Dec. 17, 2012 and titled “PWM Generation for DC/DCConverters with Frequency Switching.”

BACKGROUND OF THE INVENTION

The present disclosure relates generally to Pulse Width Modulation (PWM)controllers and more particularly to single-ended primary-inductanceconverters (SEPICs) including frequency switching functions for a PWMcontrol signal.

(PWM) is widely used to control switch mode power supplies, such as thepower supplies that are found in automotive systems. A typical approachused to generate PWM control signals uses a flip-flop, comparator and aramp generator. At the beginning of each ramp up from the rampgenerator, the output of the flip-flop is set to on, resulting in a highvoltage output. The comparator resets the flip-flop to off when theoutput of the ramp generator exceeds a predefined threshold (when theramp up exceeds the threshold). The threshold is defined by an erroramplifier that is part of a feedback control loop within the controlsystem or defined in a controller. This process repeats at a fixedfrequency generating a square wave output from the flip-flop. The squarewave output functions as the PWM control signal.

In some instances, the PWM frequency is adjusted to compensate foroperating conditions of a DC/DC converter, such as a SEPIC, used as partof the aforementioned ramp generator. Operating conditions that canrequire this adjustment are sudden changes to the input voltage of theDC/DC converter, the output voltage of the DC/DC converter, a connectedload, or any other similar operating condition.

One type of DC/DC converter that is frequently utilized in PWM systemsis a single-ended primary-inductor converter (alternately referred to asa SEPIC). PWM SEPICs typically include a compensation loop design thatkeeps the system stable when the converter is operating in adiscontinuous conduction mode. When the input voltage to the converterdecreases below a predefined threshold, the operating frequency of theconverter is decreased in order to keep a power stage of the SEPICstable. If the peak and valley values of the sawtooth signal remain thesame (i.e. the slope of the sawtooth is adjusted proportional to theratio of frequencies) then the operating frequency change causes anovershoot or undershoot. Similarly, when the frequency is increasedafter the input voltage exceeds the predefined threshold, there is acorresponding undershoot at the SEPIC output voltage.

SUMMARY OF THE INVENTION

Disclosed is a method for generating a pulse width modulation (PWM)control signal including generating a sawtooth ramp signal at a firstfrequency under standard operating conditions using a ramp generator,generating a PWM square wave having a rising edge at a falling edge ofthe sawtooth ramp signal and a falling edge when the sawtooth rampsignal exceeds an error threshold, adjusting the frequency of thesawtooth ramp in response to a changed operating parameter of the rampgenerator, and adjusting a peak input voltage of the ramp generatorsimultaneous with adjusting the frequency of the sawtooth ramp, therebypreventing one of a voltage overshoot and a voltage undershoot.

Also disclosed is a pulse width modulation (PWM) signal generatorcircuit having a voltage source, a control circuit operable to receiveinput power from the voltage source and operable to generate a sawtoothvoltage ramp signal using a ramp generator, an oscillator connected tothe ramp generator such that the oscillator controls a frequency of asawtooth ramp generated by the ramp generator, an error amplifieroperable to set an error voltage threshold, a comparator connected to anoutput of the ramp generator and an output of the error amplifier suchthat the comparator compares the output of the ramp generator againstthe output of the error amplifier, a flip flop connected to the outputof the comparator and operable to output a PWM control signal, and thecontrol circuit is operable to adjust a frequency of the ramp generatorin response to a changed operating state of a power stage, the controlcircuit including a memory storing instructions operable to cause thecontrol circuit to adjust a peak sawtooth voltage of the control circuitsimultaneously with the frequency adjustment.

These and other features of the present invention can be best understoodfrom the following specification and drawings, the following of which isa brief description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a single-ended primary-inductanceconverter (SEPIC).

FIG. 2 schematically illustrates an application specific integratedcircuit (ASIC) for a PWM generator including a SEPIC.

FIG. 3 schematically illustrates a plot of a sawtooth ramp signal and acorresponding PWM output signal maintaining a constant peak inputvoltage.

FIG. 4 illustrates a plot of a sawtooth ramp signal and a correspondingPWM output signal including adjustments on peak input voltage.

FIG. 5 illustrates a flowchart of a practical operation of a SEPIC.

DETAILED DESCRIPTION OF AN EMBODIMENT

FIG. 1 schematically illustrates a power stage 10 that operates as aDC/DC converter. The power stage 10 includes an input voltage V_(in) andan output voltage V_(out). Parallel to the input voltage V_(in) is acapacitor 20. An inductor 22 is connected to a positive terminal of thecapacitor 20, and to a positive terminal of a switch 24. The switch 24connects the first inductor 22 to a negative voltage return line 12. Asecond capacitor 26 is connected at one end to the first inductor 22,and at a second end to a second inductor 28. The second end of thesecond inductor 28 is connected to the negative voltage return line 12.A cathode end of a diode 30 is connected to the node joining the secondcapacitor 26 and the second inductor 28. A third capacitor 32 connectsthe anode end of the diode 30 to the negative voltage return line 12.The third capacitor 32 is parallel to an output voltage V_(out) of thepower stage 10.

In a typical power stage, the power stage is connected to and controlledby an Application Specific Integrated Circuit (ASIC). FIG. 2 illustratesan example ASIC 100 for controlling a power stage. The ASIC 100 includesa first voltage input 102 connected to a voltage source, a secondvoltage input 104 connected to an output voltage (V_(out)) of the powerstage, and a reference voltage input 106. The first voltage input 102 iscompared to the reference voltage 106 in a comparator 110, and theoutput of the comparator 110 controls an oscillator 120. The oscillatoroutput is passed to a ramp generator 130 that generates a sawtooth ramp.

The second voltage input 104 (V_(out)) is connected to an erroramplifier 140 that sets an error voltage (V_(e)) threshold. The erroramplifier 140 compares V_(out) to the reference voltage 106 andmagnifies the error between the values according to a known gain. In theillustrated example, a single reference voltage 106 is utilized for theASIC 100, however in alternate examples the reference voltages 106 canbe distinct voltage levels and are connected to distinct referencevoltage sources.

The output of the error amplifier 140 and the output of the rampgenerator 130 are passed to a comparator 150 that determines when theoutput of the ramp generator exceeds the error voltage (V_(e)) thresholdset by the error amplifier 140. The output of the comparator 150 and theoutput of the oscillator 120 are passed to a flip flop 160. The flipflop 160 operates in conjunction with the ramp generator 130 to output aPWM signal for controlling the power stage.

In the illustrated examples of FIGS. 1 and 2, whenever the input voltageV_(in) decreases below a predefined threshold, the operating frequencyof the power stage 10, 110 is decreased by the controller 162. Thisfrequency adjustment maintains the power stage 10, 110 in adiscontinuous mode of operations. The sawtooth voltage V_(saw) of thepower stage 10, 110 changes in this manner whenever an abrupt change inthe system input voltage occurs. These types of abrupt changes arefrequent in automotive systems.

FIG. 3 illustrates a chart 200 showing an internal sawtooth ramp voltage210 and a PWM control signal output 220 of the ASIC 100 of FIG. 2. Thechart 200 reflects a decrease in the operating frequency at time t₁resulting from an abrupt SEPIC input change. In response to the SEPICinput change, the operating frequency of the SEPIC 110 is adjusted bythe controller 162 from a first frequency (F1) to a second, lowerfrequency (F2) at time t₁ by a given factor (“K”) in order to maintainthe power stage in discontinuous mode. The given factor “K” is F1/F2. Asthe slope S1, S2 of the sawtooth ramp is a function of the frequency F1,F2 and the peak sawtooth voltage, the slope S1 of the sawtooth ramp issimilarly adjusted to a new slope S2 when the frequency changes. The newramp slope S2 is equal to the old ramp slope S1 divided by the factor K(S1/K). Thus if the original operating frequency F1 is doubled (adjustedby a factor of 2) at time t₁, the new slope S2 of the sawtooth ramp is ½of the previous slope S1.

The ASIC 100 generates a square wave pulse signal with a rising edge 222of the square wave triggered by the falling edge 212 of the sawtoothwaveform 210. The falling edge 224 of the square wave pulse signal istriggered by the sawtooth waveform 210 exceeding a pre-defined errorvoltage threshold V_(e). The error voltage threshold V_(e) is set usingany known means. In some examples the error voltage threshold V_(e) isset via the use of an error amplifier circuit incorporated into thecontroller 162 of the ASIC 100.

Assuming that the error voltage threshold V_(e) is not changed when theoperating frequency shifts from the first operating frequency F1 to thesecond operating frequency F2, maintaining a constant sawtooth voltageV_(saw) causes the duty cycle DC1 of the PWM signal at the firstfrequency F1 and the duty cycle DC2 of the PWM signal at the secondfrequency to remain the same. As is understood by those of skill in theart of PWM controls, the duty cycle of a PWM signal is the percentage ofeach period that the square wave, or pulse, is high. The equality of theduty cycles DC1, DC2 and the alteration of the slopes S1, S2 in theabove described system causes the output voltage to overshoot when theSEPIC switches to a lower operating frequency and the output voltage toundershoot when the SEPIC switches to a higher operating frequency.

In SEPIC converters operating in discontinuous mode, such as the SEPICconverters illustrated in FIGS. 1 and 2, the duty cycle is defined notonly by the input voltage V_(in) and the output voltage V_(out), butalso by converter output power. During steady state operations, theSEPIC output power combined with efficiency losses should be equal tothe SEPIC input power. When the SEPIC output power does not equal theSEPIC input power minus efficiency losses, then the SEPIC is not insteady state operations. A prime example of such a condition is duringeither a voltage undershoot or a voltage overshoot.

When the sawtooth voltage V_(saw) is fixed, such as in the previouslydescribed SEPIC, the input cycle energy is proportional to the secondpower of the ON time of the PWM signal (i.e., the second power of theduty cycle DC1, DC2). The converter output power is the product of theoutput cycle energy and the operating frequency of the power stage. Whenthe operating frequency changes from F1 to F2 by a factor of K, theoutput cycle energy also changes by a factor of K, but in the reversedirection. For example, if the second frequency F2 is twice the firstfrequency F1, then the output cycle energy is halved during the secondfrequency F2 operations.

With the above understanding, it can be appreciated that when the dutycycle DC2 at the second frequency is equal to the duty cycle DC1 at thefirst frequency, then the ON time and the peak current of the powerstage, 110 also changes by a factor of K. This, in turn, causes theinput cycle energy to be changed by a factor of K*K. Thus, when thefrequency is decreased, the input cycle energy is increased in excess ofwhat is necessary to maintain the output power. This results in avoltage overshoot for the duration of time required for the feedbackloop within the ASIC 100 to decrease the duty cycle to compensate forthe increase and the new sawtooth voltage V_(saw).

It is further appreciated that maintaining a constant duty cycle withina PWM signal is not a requirement for achieving proper PWM controls. Inthe illustrated ASIC 100 of FIG. 2, and power stage 10 of FIG. 1, theASIC 100 prevents voltage undershoots and overshoots during thisduration by adjusting the peak voltage of the sawtooth ramp using thecontroller 162. Adjusting the peak input voltage of the SEPIC 110simultaneous with adjusting the input frequency F1, F2 at t₁ preventsthe slope of the sawtooth ramp during the second frequency F2 operationsfrom being increased by the factor K, and instead adjusts the slope by adifferent factor dependent upon the new slope S2.

FIG. 4 illustrates a plot of the sawtooth ramp signal and acorresponding PWM output signal including peak input voltage adjustmentsmade by the controller 162. Unlike the PWM generation scheme illustratedin FIG. 3, when the controller 162 adjusts the frequency of the sawtoothramp signal 310, the peak input voltage of the SEPIC 110 is adjustedfrom an initial peak sawtooth voltage V_(p1) corresponding to the firstoperating frequency F1 to a second peak sawtooth voltage V_(p2)corresponding to the second operating frequency F2. By increasing thepeak sawtooth voltage V_(p1), V_(p2) alongside a decrease in thefrequency F2, the decrease in the slope of the sawtooth ramp between thefirst frequency and the second frequency is a factor of the square rootof the factor K by which the frequency was adjusted (i.e., sqrt(K))instead of being decreased by a factor of K.

In order to achieve the smaller decrease in ramp slope S1, S2 describedabove, the peak sawtooth voltage is also increased by a factor of thesquare root of the factor K by which the frequency was adjusted. Inother words, V_(p2)=V_(p1)*sqrt(K), where V_(P2) is the new peaksawtooth voltage, V_(p1) is the original peak sawtooth voltage, and K isthe factor by which the frequency is adjusted from the first frequencyF1 to the second frequency F2. The increased peak sawtooth voltageV_(p2) causes the new ramp slope S2 to be the previous ramp slope S1divided by the square root of K.

As a further result of changing the peak sawtooth voltage, the dutycycle of the PWM signal is also changed, and is not maintained constantbetween the first frequency F1 and the second frequency F2. The new dutycycle of the PWM signal at the second frequency F2 is equal to the dutycycle of the PWM signal at the first frequency F1 multiplied by thesquare root of the factor K. The utilization of multiple varied dutycycles within a single PWM signal is known in the art, and the adjustedduty cycle does not degrade the performance of the PWM signal.

As described above, when the ON time of the PWM signal and the peakcurrent of the SEPIC changes by a factor of K, the resultant input cycleenergy is changed by a factor of K*K. By adjusting the peak sawtoothvoltage V_(p1), V_(p2) in the manner described above, however, the ONtime of the PWM signal (the duty cycle DC1, DC2) is adjusted by a factorof the square root of K, and the peak current (which is directlyproportional to the peak input voltage) is similarly adjusted by afactor of the square root of K. These adjustments result in the inputcycle energy being adjusted by a factor of Sqrt(K)*Sqrt(k).Sqrt(K)*Sqrt(K)=K. Therefore, adjusting the peak sawtooth voltageV_(p1), V_(p2) in the above described manner causes the input cycleenergy to change by a factor of K, instead of a factor of K squared.This corrected adjustment maintains the steady state operations of theSEPIC and eliminates output voltage overshoot or undershoot when theoperating frequency of the SEPIC is changed.

With continued reference to FIG. 2, and with like numerals indicatinglike elements, described below, and illustrated in FIG. 5, is apractical operation of the SEPIC 110, when the SEPIC 110 undergoes afrequency adjustment. Initially, a sharp change in an operatingparameter of the SEPIC 110, such as a decrease in available inputvoltage in a “detect change in operating parameter” step. Upon detectionof the sharp increase in the input voltage the controller 162compensates for the decreased available input voltage by adjusting thefrequency of the SEPIC 110 to have a longer period in an “Adjust SEPICFrequency” step 420. Simultaneous with adjusting the SEPIC frequency,the controller 162 adjusts the peak sawtooth voltage V_(saw), allowedinto the SEPIC 110 thereby allowing the sawtooth ramp generated toexceed the original peak sawtooth voltage value V_(saw), in an “AdjustPeak V_(in),” step 430. As described above, the new sawtooth peakV_(saw) is increased by a factor of the square root of the factor bywhich the frequency was increased.

As a result of the increased frequency and the increased peak sawtoothvoltage V_(in), the duty cycle of the pulse signal generated by theflip-flop 140 and the comparator 150 self adjusts in a “Duty CycleAdjusts” step 440. The amount of the duty cycle adjustment depends onboth the SEPIC frequency adjustment and the peak sawtooth voltageV_(saw) adjustment. As the peak sawtooth voltage V_(saw) is adjusted bythe square root of the factor by which the SEPIC frequency is adjusted,the Duty Cycle is also adjusted by the square root of the factor bywhich the SEPIC frequency was adjusted.

Because the duty cycle self adjusts corresponding to the adjusted SEPICfrequency and the peak sawtooth voltage V_(saw), overshoots andundershoots of the output voltage are prevented and the SEPIC converteris maintained in the discontinuous mode.

It is further understood that any of the above described concepts can beused alone or in combination with any or all of the other abovedescribed concepts. Although an embodiment of this invention has beendisclosed, a worker of ordinary skill in this art would recognize thatcertain modifications would come within the scope of this invention. Forthat reason, the following claims should be studied to determine thetrue scope and content of this invention.

The invention claimed is:
 1. A method for generating a pulse widthmodulation (PWM) control signal comprising the steps of: generating asawtooth ramp signal at a first frequency having a first ramp slopeunder standard operating conditions using a ramp generator, the rampgenerator utilizing a single-ended primary inductance converter (SEPIC)operating in discontinuous mode; generating a PWM square wave having arising edge at a falling edge of the sawtooth ramp signal and a fallingedge when said sawtooth ramp signal exceeds an error threshold;adjusting the first frequency of the sawtooth ramp signal to a secondfrequency having a second ramp slope by a first factor in a firstdirection, which first direction is either decreasing or increasing, inresponse to a changed operating parameter of the ramp generator, thefirst factor being a ratio of the first frequency to the secondfrequency, the second ramp slope being the first ramp slope divided by asquare root of the first factor; and adjusting a peak sawtooth voltageof the ramp generator by a second factor in a second direction, thesecond factor being the square root of the first factor, and the seconddirection being opposite of the first direction, simultaneous withadjusting the first frequency of the sawtooth ramp signal to the secondfrequency, such that a duty cycle of the PWM square wave is adjusted bythe second factor in the second direction, thereby preventing one of avoltage overshoot and a voltage undershoot.
 2. The method of claim 1,wherein the changing operating parameter of the ramp generator is atleast one of an increase in a load, a decrease in a load, and a decreasein an available input power.
 3. The method of claim 1, whereinmaintaining the SEPIC converter in the discontinuous mode is achieved byutilizing a compensation loop.
 4. A pulse width modulation (PWM) signalgenerator circuit comprising: a voltage source; a control circuitoperable to receive input power from said voltage source and operable togenerate a sawtooth voltage ramp signal at a first frequency having afirst ramp slope using a ramp generator, the ramp generator utilizing asingle-ended primary-inductance converter (SEPIC) configured to operatein discontinuous mode; an oscillator connected to the ramp generatorsuch that said oscillator controls the first frequency of the sawtoothvoltage ramp signal generated by the ramp generator; an error amplifieroperable to set an error voltage threshold; a comparator connected to anoutput of the ramp generator and an output of the error amplifier suchthat said comparator compares the output of the ramp generator againstthe output of the error amplifier; a flip-flop connected to the outputof the comparator and operable to output a PWM control signal; whereinthe control circuit is operable to adjust the first frequency of thesawtooth voltage ramp signal of the ramp generator to a second frequencyhaving a second ramp slope in response to a changed operating state of apower stage, the control circuit including a memory storing instructionsoperable to cause the control circuit to adjust a peak sawtooth voltageof the control circuit simultaneously with the frequency adjustment; andwherein said control circuit is operable to adjust the peak sawtoothvoltage of the ramp generator by a first factor in a first direction,which first direction is either decreasing or increasing, and whereinsaid control circuit is operable to adjust the first frequency of thesawtooth voltage ramp signal of the ramp generator to a second frequencyby a second factor in a second direction, wherein the first factor is asquare root of the second factor, and the second direction is oppositeof the first direction, and wherein a duty cycle of a generated PWMsignal is adjusted by the first factor in the first directionsimultaneous with the frequency adjustment, the second factor being aratio of the first frequency to the second frequency, and the secondramp slope being the first ramp slope divided by the square root of thesecond factor.
 5. The PWM signal generator circuit of claim 4, whereineach of said oscillator, comparator, error amplifier and said flip-flopare components of an application specific integrated circuit (ASIC).